The disclosure of Japanese Patent Application No. 2000-213702 filed Jul. 14, 2000 including the specification, drawings and abstract is incorporated by reference in its entirety.
1. Field of Invention
The invention relates to vehicle speed estimating apparatus and method of a motor vehicle, and, more particularly, to vehicle speed estimating apparatus and method for estimating a vehicle speed based on wheel speeds, and a vehicular control apparatus that performs control based on the estimated vehicle speed.
2. Description of Related Art
A known example of vehicle speed estimating apparatus for estimating a vehicle speed of a four-wheel drive (4WD) vehicle is disclosed in U.S. Pat. No. 5,913,377 (corresponding to JP-A-10-230837). The known vehicle speed estimating apparatus estimates a vehicle speed based on the lowest one of wheel speeds of all of the wheels. In a two-wheel-drive (2WD) vehicle, it is known to estimate a vehicle speed based on the wheel speeds of non-driven wheels.
According to the vehicle speed estimating apparatus and method as described above, the vehicle speed is estimated based on the wheel speed(s) that is/are less likely to be influenced by a slip of a wheel or wheels. Therefore, the vehicle speed can be estimated with improved accuracy as compared with the case where the vehicle speed is estimated without taking the influence of a slip of the wheel(s) into account.
In some types of four-wheel drive vehicles, the driving state can be switched between a 4WD state and a 2WD state. In order to accurately estimate a vehicle speed with minimum influence caused by any slip of a wheel(s), it is necessary to determine whether the vehicle is in the 4WD state or the 2WD state, and select the wheel speed(s) as the basis for the estimated vehicle speed, depending upon the result of the determination. Accordingly, if the result of determination on the driving state of the vehicle differs from the actual driving state, or if the driving state cannot be identified or is not clear, the vehicle speed cannot be estimated with high accuracy.
For example, when the vehicle is actually in the 4WD state, but the vehicle speed estimating apparatus determines that the vehicle is in the 2WD, state, i.e., due to a failure or abnormality in a signal communication system or a driving state switching device, the vehicle speed is estimated based on the wheel speeds of the wheels that are actually driven wheels but are regarded as non-driven wheels, even though the vehicle speed must be estimated based on the lowest one of the wheel speeds of all of the wheels. As a result, the vehicle speed is estimated to be higher than an actual value thereof.
The invention was developed in view of the above-described problem encountered in vehicles in which the driving state can be switched between the 4WD and 2WD states, in the case where a vehicle speed is estimated according to the known vehicle speed estimating apparatus or method. Thus, the invention mainly aims at estimating a vehicle speed with improved accuracy even when the actual driving state of the vehicle is different from a result of determination on the driving state, or when the driving state of the vehicle cannot be identified or is not clear. Accordingly, the estimating apparatus and method according to the invention estimates a vehicle speed when the driving state is not determined as a selected-wheel driving state, e.g., 2WD state, in which some of the wheels are driven wheels, in the same manner as in the case where the driving state is determined as an all-wheel driving state, e.g., 4WD state, in which all of the wheels are driven wheels.
For example, there are situations where certain control is performed using the estimated vehicle speed in a vehicle in which the driving state can be switched between the 4WD state and the 2WD state. The control may be, for example, braking-force behavior control under which a braking force is applied to a certain wheel or wheels when the vehicle behaves in an unfavorable or undesirable fashion, so as to stabilize the vehicle behavior. In this case, if the vehicle speed is estimated to be higher than an actual value thereof, the behavior control may be undesirably applied in a situation where it should not be applied, or may fail to be terminated in a situation where it should be terminated. Another example of control is traction control that is executed upon an excessive slip of a wheel or wheels, for suppressing the slip by applying braking force to the wheel(s). If the vehicle speed is estimated to be higher than an actual value thereof, and the traction control is performed based on the estimated vehicle speed, the traction control may not be started in a situation where it should be started, or may be terminated in a situation where it should not be terminated.
It is therefore an object of the invention to provide an apparatus and a method for estimating a vehicle speed with improved accuracy based on wheel speeds, even when the actual driving state of the vehicle is different from a result of determination on the driving state, or when the driving state of the vehicle cannot be identified or is not clear.
It is another object of the invention to provide vehicular control apparatus and method for performing predetermined control more appropriately, based on the estimated vehicle speed.
To accomplish the above and/or other object(s), one aspect of the invention provides an apparatus for estimating a vehicle speed of a vehicle in which a driving state can be switched between an all-wheel driving state, i.e., all of the wheels of the vehicle are driven wheels, and a selected-wheel driving state in which a first group of wheels are driven wheels and a second group of wheels are non-driven wheels. A controller of the apparatus determines the driving state of the vehicle, and calculates the vehicle speed based on a wheel speed of at least one of the non-driven wheels when it is determined that the driving state is the selected-wheel driving state. When it is determined that the driving state is not the selected-wheel driving state or when it cannot be determined that the driving state is the selected-wheel driving state, the controller calculates the vehicle speed based on at least a wheel speed of at least one of the wheels of the vehicle. The number of the at least one of the wheels is equal to or smaller than that of the non-driven wheels, as represented by xe2x80x9cNxe2x80x9d, and the at least one of the wheels has the lowest to the Nth lowest one of the wheel speeds of all of the wheels.
The vehicle speed estimating apparatus is able to estimate the vehicle speed with improved accuracy, when the vehicle is actually in the all-wheel driving state, but the controller determines that the vehicle is in the selected-wheel driving state due to, for example, a failure or abnormality in a signal communication system, or a driving state switching device, or the like. In this case, the vehicle speed is estimated based on at least a wheel speed of at least one wheel whose number is equal to or smaller than that of non-driven wheels as represented by xe2x80x9cNxe2x80x9d, and which has the lowest to the Nth lowest one of the wheel speeds of all of the wheels. Accordingly, a situation can be avoided where the vehicle speed is estimated based on the speeds of wheels that are actually driven wheels but are falsely judged as non-driven wheels, and where the vehicle speed is estimated to be higher than an actual value because of the misjudgment.
Another aspect of the invention provides an apparatus and a method for performing predetermined control on a vehicle in which a driving state can be switched between an all-wheel driving state in which all of the wheels of the vehicle are driven wheels, and a selected-wheel driving state in which a first group of wheels are driven wheels and a second group of wheels are non-driven wheels. A controller of the apparatus estimates the vehicle speed in the same manner as described above with respect to the vehicle speed estimating apparatus and method, and performs the predetermined control by using the estimated vehicle speed.
The control apparatus constructed according to the above aspect of the invention is able to estimate the vehicle speed with improved accuracy, as in the case of the vehicle speed estimating apparatus as described above. Thus, there are reduced possibilities that a predetermined control is unexpectedly applied in a situation where it should not be applied, the predetermined control fails to be terminated in a situation where it should be terminated, the predetermined control fails to be applied when it should be applied, and the predetermined control is unexpectedly terminated when it should not be terminated.